common: don't use dllexport when GVDLL and _BLD_gvc are set

common: don't use dllexport when GVDLL and _BLD_gvc are set

This is a temporary measure until an upcoming commit series cleans up
the control of the Windows storage-class attributes.

Without this change, an upcoming commit that defines GVDLL would cause
many warnings like this:

globals.obj : warning LNK4197: export 'Version' specified multiple times; using first specification [C:\Users\magja\graphviz\build\lib\gvc\gvc.vcxproj]

The reason for this is that the symbols are already defined in the
gvc.def file. For more, see
https://support.microsoft.com/en-us/topic/you-receive-an-lnk4197-error-in-the-64-bit-version-of-the-visual-c-compiler-f002b07e-4ba6-46d4-eed4-0c82412b4ac6.